A so-called GTL process for producing liquid hydrocarbons that contain fuel oil from natural gas is described, for example, in WO 2007/114274 A1. FIG. 3 of the accompanying drawings is a schematic illustration of the flow of such a known GTL process.
The GTL process illustrated in FIG. 3 includes a hydrodesulfurization step 120 of hydrodesulfurizing sulfur compounds in natural gas, a synthesis gas production step 130 of producing synthesis gas by way of a reforming reaction of natural gas with steam and/or carbon dioxide, a carbon dioxide removal step 140, which is provided whenever necessary, a Fischer-Tropsch oil production step 150 of producing Fischer-Tropsch oil from the synthesis gas by way of Fischer-Tropsch (FT) synthesis, an upgrading reaction step 160 of hydrogenating the produced Fischer-Tropsch oil and an upgrading gas/liquid separation step 170 of subjecting the hydrogenated product obtained by the upgrading reaction step to gas/liquid separation to obtain liquid hydrocarbons, the above steps being arranged sequentially from the natural gas feed side (or the upstream side) or from the left side in FIG. 3.
The synthesis gas produced from the synthesis gas production step 130 partly branches off at a stage prior to getting to the Fischer-Tropsch oil production step 150 to form a branch line 145 and the synthesis gas in the branch line 145 is separated into high-purity hydrogen (line 192) and purge gas (line 191) in a hydrogen separation step 190 typically by means of a hydrogen PSA (pressure swing adsorption) method. The separated high-purity hydrogen partly joins a hydrogen circulation line 177, by way of lines 192 and 197, where hydrogen circulates from the upgrading gas/liquid separation step 170 to the upgrading reaction step 160, while the remaining part is supplied to a hydrodesulfurization step through a line 196. On the other hand, the purge gas that is purged from the line 191 is normally consumed as fuel.
In the above-described known process, the concentration of hydrogen supplied to the upgrading reaction step 160 is about 92 mol %. If the concentration of hydrogen supplied to the upgrading reaction step 160 can be raised from the level of the prior art, the pressure required for the hydrogenation reaction of the upgrading reaction step 160 can be reduced to reduce the operation cost. Additionally, the reaction efficiency is improved to make it possible to reduce the size of the reactor for the upgrading reaction step 160. Furthermore, the step can be conducted at a lower reaction temperature to suppress deactivation of the catalyst and increase the catalyst life.
Since the purge gas 191 discharged from the hydrogen separation step 190 contains unreacted methane, a significant improvement can be achieved in terms of raw material consumption per product to an economic advantage if it can be taken into the process once again and reutilized as raw material.
However, there is not any technique proposed to date to dissolve the above-identified problem and realize a realistic process for treating such purge gas.